The goal of this research is to understand how the functional architecture of the speech-lexical processing system maps on to the neural systems underlying it using event-related functional magnetic resonance imaging (fMRI). Two hypotheses will be investigated, both exploring the neural systems underlying the computational properties of auditory speech and lexical processing. The first hypothesis explores whether the computational mechanisms of the two hemispheres preferentially process different properties of the speech signal. A series of experiments using both speech discrimination and phonetic categorization tasks will examine a number of acoustic-phonetic parameters underlying both consonant and vowel perception including voice-onset time and formant frequency duration in the perception of consonants, and duration and formant frequency (F l-F2) space in the perception of vowels. It is hypothesized that the acoustic properties corresponding to the phonetic categories of speech will be preferentially processed by the left hemisphere irrespective of their acoustic structure owing to the functional role that sound structure plays in language processing. In contrast, it is hypothesized that activation of the right hemisphere will be modulated by the inherent acoustic properties of the speech signal. The second hypothesis explores how the computational properties of the speech lexical processing system map on to its neural substrate. It is hypothesized that there will be increased left frontal activation under conditions of phonetic-phonological, lexical, and/or semantic competition. Phonetic competition will be investigated by exploring the extent to which acoustic-phonetic structure and the proximity of an exemplar to a contrasting, and hence, competing phonetic category influences frontal activation. Lexical competition will be investigated by exploring the influence of phonetic factors on lexical competition and phonological factors on lexical neighborhood density. Semantic competition will be investigated by exploring the effects of semantically ambiguous words compared to unambiguous words on neural activation patterns.